Array waveguide grating, array waveguide grating module, optical communication unit and optical communication system

ABSTRACT

First channel waveguides  102   1  to  102   3  of an array waveguide grating are connected via a first to a third exponential function shape optical waveguide  111   1  to  111   3  to a first sector-shape slab waveguide  105 . In a second boundary part  109  which is disposed symmetrically with a first boundary part  108  via a channel waveguide array  104 , second channel waveguides  103   1  to  103   3  are connected via a first to a third taper shape optical waveguide  112   1  to  112   3  to a second sector-shape slab waveguide  106 . By adopting exponential function shape optical waveguides  111  at least partly, the optical frequency characteristics can be improved compared to the case of the second degree function shape, and also the degree of freedom can also be improved compared to the case of the rectangular shape.

The present Application is a Divisional Application of U.S. patentapplication Ser. No. 10/121,678, filed on Apr. 15, 2002.

BACKGROUND OF THE INVENTION

This application claims benefit of Japanese Patent Application No.2001-116749 filed on Apr. 16, 2001, the contents of which areincorporated by the reference.

The present invention relates to array waveguide gratings used as lightwavelength multiplexing/demultiplexing elements for opticalcommunication, array waveguide grating modules, optical communicationunits and optical communication systems using the same array wavelengthlattices. More specifically, the present invention concerns arraywaveguide gratings with improved light signal frequency characteristics,array waveguide modules, optical communication units and opticalcommunication systems using the same array waveguide gratings.

With processes of usual time internet connection and communication datacapacity increase, demands for large capacity data transfer areincreasing. In the optical communication using light signals, it is veryimportant for large capacity data transfer to improve the degree ofwavelength multiplexing. In this respect, the role of array waveguidegratings as multiplexing/demultiplexing elements for multiplexing anddemultiplexing light wavelengths is important, and the array waveguidegratings are thought to be one of key devices. The array waveguidegrating has a passive structure, and also has a narrow light wavelengthtransmission width and a high extinction ratio. The array waveguidegrating also has such features as that it can multiplex and demultiplexa number of light signals in correspondence to the number of waveguides.

Such array waveguide grating is desirably free from sudden changes ofits output level or loss level with variations of the laser output lightsignal frequency from the center optical frequency of each opticalwaveguide. Also, where multiple stages of array waveguide gratings areconnected, the modulation components of the light signal are cut offoutside a bandwidth, in which the individual array waveguide gratingscommonly transmit the light signal. Thus, it is important from thestandpoint of improving the light signal transmission efficiency as wellto realize a transmission characteristic with a flat peak level withrespect to optical frequency.

FIG. 33 shows an example of such array waveguide grating. Theillustrated array waveguide grating 10 has a substrate 11, on which oneor more first channel waveguides (i.e., input channel waveguides) 12, aplurality of second channel waveguides (i.e., output channel waveguides)13, a channel waveguide array 14 with a plurality of component channelwaveguides bent in a predetermined direction with different radii ofcurvature, a first sector-shape slab waveguide 15 connecting the firstchannel waveguides 12 and the channel waveguide array 14 to one anotherand a second sector-shape slab waveguide 16 connecting the channelwaveguide array 14 and the second channel waveguides 13 to one another,are formed. Multiplexed light signals with wavelengths λ₁ to λ_(n), areincident from the first channel waveguides 12 on the first sector-shapeslab waveguide 15, then proceed with their paths expanded therethoughand are then incident on the channel waveguide array 14.

In the channel waveguide array 14, the component array waveguides haveprogressively increasing or reducing optical path lengths with apredetermined optical path length difference provided between adjacentones of them. Thus, the light beams proceeding through the individualarray waveguides reach the second sector-shape slab waveguide 16 with apredetermined phase difference provided between adjacent ones of them.Actually, wavelength dispersion takes place, and the in-phase plane isinclined in dependence on the wavelength. Consequently, the light beamsare focused (i.e., converged) on the boundary surface between the secondsector-shape slab waveguide 16 and the plurality of second channelwaveguides 13 at positions different with wavelengths. The secondchannel waveguides 13 are disposed at positions corresponding to theirrespective wavelengths. Given wavelength components λ₁ to λ_(n) thus canbe taken out independently from the second channel waveguides 13.

FIG. 34 shows, to an enlarged scale, a boundary part between the firstchannel waveguides and the first sector-shape slab waveguide in thearray waveguide grating shown in FIG. 33. The first channel waveguides121 to 123, which are shown in a first boundary part 18 shown in FIG. 33as well, have optical waveguides 211 to 213 having a rectangular shapewith a width Wp and length L2 and terminating in the first sector-shapeslab waveguide 15.

FIG. 35 shows a boundary part in the case of using parabolic or seconddegree function shape waveguides disclosed in Japanese Patent Laid-OpenNo. 9-297228. In this case, the first channel waveguides 121 to 123shown in the first boundary part 18 have optical waveguides 221 to 223having a second degree function shape with a length L2 and terminatingwith a width Wp in the sector-shape slab waveguide 15.

Insertion loss and transmission width are usually in a trade-offrelation to each other. However, where rectangular optical waveguides211 to 213 shown in FIG. 34 are used in lieu of the second degreefunction shape light waveguides 221 to 223 shown in FIG. 35, thetransmission width can be improved without sacrifice in the insertionloss. It is thus a great merit to use the rectangular optical waveguides211 to 213 shown in FIG. 34 for realizing a flat transmitted lightfrequency characteristic.

The above description has concerned with the shapes of the opticalwaveguides, which are disposed in the first boundary part 18 between thefirst channel waveguide 12 and the first sector-shape slab waveguide 15shown in FIG. 33. Such optical waveguides 21 and 22 are provided for thepurpose of providing for harmonic mode of input at their locality ofcontact with the slab waveguide to make the Gaussian waveform peak partas flat as possible.

In lieu of providing the above contrivance with respect to the opticalwaveguides 21 and 22, the same effects are obtainable by providingoptical waveguides of the same shapes in the second boundary part 19 asthe boundary between the second channel waveguides 13 and the secondsector-shape slab waveguide 16. Here, for the sake of the simplicity ofdescription, only the shapes of the optical waveguides in the firstboundary part 18 will be considered.

Where the rectangular optical waveguides 211 to 213 as shown in FIG. 34are used, the variable shape parameters are only the width Wp and thelength L2 of the rectangular part. Therefore, if the width Wp and thelength L2 can assume only values limited on the design, it is possibleto change the characteristics in such ranges. In other words, in thiscase a problem is posed that the degree of freedom in fine adjustmentand fine design for realizing various properties is very low. Forexample, the problem may concern the transmission width and the strokein the trade-off relation to each other. These problems will bediscussed in detail in the following.

FIG. 36 shows an ideal characteristic of wavelength multiplexed lightsignals. In the graph, the ordinate is taken for the transmitted lightsignal power level, and the abscissa is taken for the wavelength. Theindividual light signals 311, 312, 313 have a rectangular waveform andalso have a maximum transmission width. Thus, signal components of otherlight signals are not mixed with the signal components of the intrinsiclight signals. Where such ideal light signals 311 to 313 aremultiplexed, by connecting multiple stages of array waveguide gratingsor array waveguide grating modules the bandwidth of the individual lightsignals is not reduced. The center wavelength of the light signals 311to 313 may be deviated, but the signal level is not varied. However, nolight signal transmitted through such array waveguide grating has suchideal rectangular waveform.

FIG. 37 shows a summary of proposal of an array waveguide grating with arectangular optical waveguide connected to a slab waveguide. In theFigure, parts like those in FIG. 33 are designated by like referencenumerals and symbols. In this proposal, first channel waveguide 12 andfirst sector-shape slab waveguide 15 are connected to each other by arectangular waveguide 33.

FIG. 38 shows a way of use of the array waveguide grating shown in FIG.37 such that multiplexed light signal is spread as it is led fromchannel waveguide through rectangular optical waveguide and then takenout as light signals each separated for each wavelength. As a lightsignal 32 passes through a rectangular optical waveguide 33, it ischanged to a harmonic mode light signal 34 and spread. The spread lightsignal is converged through a channel waveguide array 14 and atpositions each peculiar for each wavelength. The converged light signal37 is separated and taken out for each wavelength in such a form as tocorrespond to the position of a second channel waveguide 13.

FIG. 39 shows optical frequency characteristics of light signals takenout in the example shown in FIG. 38. As shown, individual light signals37 are multiplexed with a high density, and skirt portions of adjacentlight signals and also skirt portions of light signals at spaced-apartpositions are complicatedly intrude in the wavelength ranges ofintrinsic light signals.

FIG. 40 shows light signals of two adjacent channels. Light signals 331and 332 shown by solid curves have a smaller transmission width T asshown by arrows than the case of light signals 341 and 342 shown bybroken lines, but the influence of noise components due to cross-talk isless. However, the light signals 331 and 332 are sharper in waveformthan the light signals 341 and 342, and therefore they are subject togreater loss in the case of deviation from the center wavelength. Asshown, the optical frequency characteristic varies with the light signalwaveform shape. For this reason, when building a communication system,it is necessary to determine the optical frequency characteristic of thearray waveguide grating or the array waveguide grating module on thebasis of a desire of giving preference to the transmission width orattaching importance to the cross-talk. For example, in the case of atrunk communication system it is possible that light signal is relayedat many places as it is transferred, and it is thought to attachimportance to the cross-talk for minimizing the deterioration of signal.In the case of a terminal communication system, on the other hand,simpler circuit devices than those in the trunk system are used. In thiscircumstance, a certain extent of deviation from the center wavelengthof each signal channel has to be allowed. In this case, importance thusmay be attached to the transmission width.

Thus, as described before, with the rectangular optical waveguides 211to 213 as shown in FIG. 34 the degree of freedom of changing the opticalfrequency characteristics in dependence on the circumstance with thearray waveguide grating used therein is low. In this respect, theoptical waveguides 221 to 223 having the second degree function shape asshown in FIG. 35 become attractive.

However, the wavelength multiplexing degree improvement demand is on atrend of being increased more and more. When the channel width of eachlight signal is correspondingly reduced, the gap width between thesignal transmission widths of adjacent channel light signals arerelatively reduced to strengthen the degree of inter-channelinterference, thus resulting in relative cross-talk deterioration. Inthis situation, it is difficult to manufacture array waveguide gratingsor array waveguide grating modules, which permit satisfactorily settingtransmission width and cross-talk for meeting demands for variouscommunication systems.

SUMMARY OF THE INVENTION

An object of the present invention, therefore, is to provide arraywaveguide gratings, array waveguide grating modules and opticalcommunication systems capable of increasing the degree of freedom ofoptical frequency characteristics and obtaining transmitted light offlatter characteristics than in the case where second degree functionshape optical waveguides are used for connecting channel waveguide andslab waveguide to one another.

According to a first aspect of the present invention, there is providedan array waveguide grating comprising: a predetermined substrate; afirst and a second channel waveguide for light wave transfer on thesubstrate; a channel waveguide array having a plurality of componentwaveguides having lengths progressively increasing with a predetermineddifference between adjacent ones of the waveguides on the substrate; afirst slab waveguide for connecting the ends of the first channelwaveguides and one end of the channel waveguide array via a waveguidepart having a first shape on the substrate; and a second slab waveguidefor connecting one end of the second channel waveguides and the otherend of the channel waveguide array via a waveguide part having a secondshape on the substrate; wherein: at least the open part of each of thefirst channel waveguides on the side of the first slab waveguide or theopen part of each of the second channel waveguides on the side of thesecond slab waveguide is flaring in an exponential function shape towardthe channel waveguide array.

According to a second aspect of the present invention, there is providedan array waveguide grating comprising: a predetermined substrate; afirst and a second channel waveguide for light wave transfer on thesubstrate; a channel waveguide array having a plurality of componentwaveguides having lengths progressively increasing with a predetermineddifference between adjacent ones of the waveguides on the substrate; afirst slab waveguide for connecting the ends of the first channelwaveguides and one end of the channel waveguide array via a waveguidepart having a first shape on the substrate; and a second slab waveguidefor connecting one end of the second channel waveguides and the otherend of the channel waveguide array via a waveguide part having a secondshape on the substrate; wherein: at least a part of at least the openpart of each of the first channel waveguides on the side of the firstslab waveguide or the open part of each of the second channel waveguideson the side of the second slab waveguide is flaring in an exponentialfunction shape toward the channel waveguide array.

In a third aspect of the present invention according to the first orsecond aspect, the shape W(X) flaring in the exponential function shapeis represented asW(X)=(Wp−Wc)*(1−exp(−a*X))+Wcwhere X represents the light wave progress direction, Wp is the width ofthe end of the shape connected to the slab waveguide, Wc is the spreadof the waveguide part in directions perpendicular to the light waveprogress direction X, and a represents a parameter (i.e., shapevariable) giving the exponential function shape.

In a fourth aspect of the present invention according to the thirdaspect, wherein the shape variable a giving the exponential functionshape is unity or below.

In a fifth aspect of the present invention according to the thirdaspect, both of the first and second shape waveguide parts have a shapepart flaring from in an exponential function shape from the side of thechannel waveguides toward the channel waveguide array and are differentin the value of the shape variable a from each other.

In a sixth aspect of the present invention according to the thirdaspect, at least either the open part of each of the first channelwaveguides on the side of the first slab waveguide or the open part ofeach of the second channel waveguides on the side of the second slabwaveguide has a shape part flaring in an exponential function shapetoward the channel waveguide array, and the value of the shape variablea is set independently to a value corresponding to a correspondingchannel waveguide.

In a seventh aspect of the present invention according to the secondaspect, wherein parts of the first and second shape waveguide partswhich do not have any shape part flaring in the exponential functionshape have a taper shape.

In an eighth aspect of the present invention according to the secondaspect, parts of the first and second shape waveguide parts which do nothave any shape part flaring in the exponential function shape have asecond degree function shape.

In a ninth aspect of the present invention according to the secondaspect, parts of the first and second shape waveguide parts which do nothave any shape part flaring in the exponential function shape have botha taper shape and a second degree function shape. In a tenth aspect ofthe present invention according to the second aspect, the other shapesin the case of a part containing a shape part flaring in the exponentialfunction shape consist of a taper shape part. In an eleventh aspect ofthe present invention according to the second aspect, the other shapesin the case of a part containing a shape part flaring in the exponentialfunction shape consist of a second degree function shape part.

In a twelfth aspect of the present invention according to the secondaspect, the other shapes in the case of a part containing a shape partflaring in the exponential function shape consist of a taper shape partand a second degree function shape part.

According to a thirteenth aspect of the present invention, there isprovided an array waveguide grating comprising: a predeterminedsubstrate; a first and a second channel waveguide for light wavetransfer on the substrate; a channel waveguide array having a pluralityof component waveguides having lengths progressively increasing with apredetermined difference between adjacent ones of the waveguides on thesubstrate; a first slab waveguide for connecting the ends of the firstchannel waveguides and one end of the channel waveguide array via awaveguide part having a first shape on the substrate; and a second slabwaveguide for connecting one end of the second channel waveguides andthe other end of the channel waveguide array via a waveguide part havinga second shape on the substrate; wherein: at least the open part of eachof the first channel waveguides on the side of the first slab waveguideor the open part of each of the second channel waveguides with respectto the second slab waveguide has a shape part flaring in an exponentialfunction shape represented by a function of a degree higher than thesecond degree toward the channel waveguide array.

According to a fourteenth aspect of the present invention, there isprovided an array waveguide grating comprising: first and second channelwaveguides for light wave transfer; a channel waveguide array having aplurality of component waveguides having lengths progressivelyincreasing with a predetermined difference between adjacent ones of thewaveguides; a first slab waveguide disposed between the first channelwaveguides and one end of the channel waveguide array; and a second slabwaveguide disposed between the second channel waveguides and the otherend of the channel waveguide array; wherein: at least the open part ofeach of the first channel waveguides on the side of the first slabwaveguide or the open part of each of the second channel waveguides onthe side of the second slab waveguide has an open end with an openingwidth greater than the waveguide width of the first or second channelwaveguides; and the shape directed from the stem part of the open parttoward the open end is found on the inner side of rectangular shape ofthe opening width and on the outer side of a second degree curveconnecting the stem part and the open end.

In a fifteenth aspect of the present invention according to thethirteenth aspect, the flaring shape part represented by the function ofa degree higher than the second degree has such a convex shape that whenfrequency multiplexed Gaussian waveform light waves pass through theirwaveguides, their characteristics line in a rage between boundary rangesof characteristics with respect to the transmission width and thecross-talk when they pass through the rectangular waveguides and seconddegree function shape waveguides.

According to a sixteenth aspect of the present invention, there isprovided an array waveguide grating module comprising: an arraywaveguide grating including a predetermined substrate, a first and asecond channel waveguide for light wave transfer on the substrate, achannel waveguide array having a plurality of component waveguideshaving lengths progressively increasing with a predetermined differencebetween adjacent ones of the waveguides on the substrate, a first slabwaveguide for connecting the ends of the first channel waveguides andone end of the channel waveguide array via a waveguide part having afirst shape on the substrate, and a second slab waveguide for connectingone end of the second channel waveguides and the other end of thechannel waveguide array via a waveguide part having a second shape onthe substrate, wherein at least the open part of each of the firstchannel waveguides on the side of the first slab waveguide or the openpart of each of the second channel waveguides on the side of the secondslab waveguide is flaring in an exponential function shape toward thechannel waveguide array; and an optical fiber having one end opticallyconnected to at least part of the first or second channel waveguides ofthe array waveguide grating.

According to a seventeenth aspect of the present invention, there isprovided an array waveguide grating module comprising: an arraywaveguide grating including a predetermined substrate, a first and asecond channel waveguide for light wave transfer on the substrate, achannel waveguide array having a plurality of component waveguideshaving lengths progressively increasing with a predetermined differencebetween adjacent ones of the waveguides on the substrate, a first slabwaveguide for connecting the ends of the first channel waveguides andone end of the channel waveguide array via a waveguide part having afirst shape on the substrate, and a second slab waveguide for connectingone end of the second channel waveguides and the other end of thechannel waveguide array via a waveguide part having a second shape onthe substrate; wherein at least a part of at least the open part of eachof the first channel waveguides on the side of the first slab waveguideor the open part of each of the second channel waveguides on the side ofthe second slab waveguide is flaring in an exponential function shapetoward the channel waveguide array; and an optical fiber having one endoptically connected to at least part of the first or second channelwaveguides of the array waveguide grating.

According to an eighteenth aspect of the present invention, there isprovided an array waveguide grating module comprising: an arraywaveguide grating including first and second channel waveguides forlight wave transfer, a channel waveguide array having a plurality ofcomponent waveguides having lengths progressively increasing with apredetermined difference between adjacent ones of the waveguides, afirst slab waveguide disposed between the first channel waveguides andone end of the channel waveguide array and a second slab waveguidedisposed between the second channel waveguides and the other end of thechannel waveguide array, wherein at least the open part of each of thefirst channel waveguides on the side of the first slab waveguide or theopen part of each of the second channel waveguides on the side of thesecond slab waveguide has an open end with an opening width greater thanthe waveguide width of the first or second channel waveguides, and theshape directed from the stem part of the open part toward the open endis found on the inner side of rectangular shape of the opening width andon the outer side of a second degree curve connecting the stem part andthe open end; and an optical fiber having one end optically connected toat least part of the first or second channel waveguides of the arraywaveguide grating.

In a nineteenth aspect of the present invention according to thesixteenth or seventeenth aspect, the shape W(X) flaring in theexponential function shape is represented asW(X)=(Wp−Wc)*(1−exp(−a*X))+Wcwhere X represents the light wave progress direction, Wp is the width ofthe end of the shape connected to the slab waveguide, Wc is the spreadof the waveguide part in directions perpendicular to the light waveprogress direction X, and a represents a parameter (i.e., shapevariable) giving the exponential function shape.

In a twentieth aspect of the present invention according to thenineteenth aspect, the shape variable a giving the exponential functionshape is unity or below.

In a twenty-first aspect of the present invention according to thenineteenth aspect, both of the first and second shape waveguide partshave a shape part flaring from in an exponential function shape from theside of the channel waveguides toward the channel waveguide array andare different in the value of the shape variable a from each other.

In a twenty-second aspect of the present invention according to thenineteenth aspect, at least either the open part of each of the firstchannel waveguides on the side of the first slab waveguide or the openpart of each of the second channel waveguides on the side of the secondslab waveguide has a shape part flaring in an exponential function shapetoward the channel waveguide array, and the value of the shape variablea is set independently to a value corresponding to a correspondingchannel waveguide.

In a twenty-third aspect of the present invention according to thesixteenth or seventeenth aspect, parts of the first and second shapewaveguide parts which do not have any shape part flaring in theexponential function shape have a taper shape.

In a twenty-fourth aspect of the present invention according to thesixteenth or seventeenth aspect, parts of the first and second shapewaveguide parts which do not have any shape part flaring in theexponential function shape have a second degree function shape.

In a twenty-fifth aspect of the present invention according to thesixteenth or seventeenth aspect, parts of the first and second shapewaveguide parts which do not have any shape part flaring in theexponential function shape have both a taper shape and a second degreefunction shape.

In a twenty-sixth aspect of the present invention according to theseventeenth aspect, the other shapes in the case of a part containing ashape part flaring in the exponential function shape consist of a tapershape part.

In a twenty-seventh aspect of the present invention according to theseventeenth aspect, the other shapes in the case of a part containing ashape part flaring in the exponential function shape consist of a seconddegree function shape part.

In a twenty-eighth aspect of the present invention according to theseventeenth aspect, the other shapes in the case of a part containing ashape part flaring in the exponential function shape consist of a tapershape part and a second degree function shape part.

In a twenty-ninth aspect of the present invention, there is provided anarray waveguide grating module comprising: an array waveguide gratingincluding a predetermined substrate, a first and a second channelwaveguide for light wave transfer on the substrate, a channel waveguidearray having a plurality of component waveguides having lengthsprogressively increasing with a predetermined difference betweenadjacent ones of the waveguides on the substrate, a first slab waveguidefor connecting the ends of the first channel waveguides and one end ofthe channel waveguide array via a waveguide part having a first shape onthe substrate, and a second slab waveguide for connecting one end of thesecond channel waveguides and the other end of the channel waveguidearray via a waveguide part having a second shape on the substrate,wherein at least the open part of each of the first channel waveguideson the side of the first slab waveguide or the open part of each of thesecond channel waveguides with respect to the second slab waveguide hasa shape part flaring in an exponential function shape represented by afunction of a degree higher than the second degree toward the channelwaveguide array; and an optical fiber having one end optically connectedto at least part of the first or second channel waveguides of the arraywaveguide grating.

In a thirty aspect of the present invention according to thetwenty-ninth aspect, the flaring shape part represented by the functionof a degree higher than the second degree has such a convex shape thatwhen frequency multiplexed Gaussian waveform light waves pass throughtheir waveguides, their characteristics line in a rage between boundaryranges of characteristics with respect to the transmission width and thecross-talk when they pass through the rectangular waveguides and seconddegree function shape waveguides.

According to a thirty-first aspect of the present invention, there isprovided an optical communication system comprising: an opticaltransmission means for sending out light signals of differentwavelengths as parallel signals; a multiplexer constituted by an arraywaveguide grating for wavelength multiplexing/demultiplexing each of thedifferent wavelength light signals sent out from the opticaltransmission means; an optical transmission line, to which thewavelength divided and multiplexed light signals outputted from themultiplexer are sent; a node provided in the optical transmission lineand having an array waveguide grating; a demultiplexer constituted by anarray waveguide array for receiving input light signal set along theoptical transmission line via the node; and an optical receiving meansfor receiving the demultiplexed different wavelength light signals fromthe demultiplexer; wherein the demultiplexer includes a predeterminedsubstrate, a first and a second channel waveguide for light wavetransfer on the substrate, a channel waveguide array having a pluralityof component waveguides having lengths progressively increasing with apredetermined difference between adjacent ones of the waveguides on thesubstrate, a first slab waveguide for connecting the ends of the firstchannel waveguides and one end of the channel waveguide array via awaveguide part having a first shape on the substrate, and a second slabwaveguide for connecting one end of the second channel waveguides andthe other end of the channel waveguide array via a waveguide part havinga second shape on the substrate, and at least the open part of each ofthe first channel waveguides on the side of the first slab waveguide orthe open part of each of the second channel waveguides on the side ofthe second slab waveguide is flaring in an exponential function shapetoward the channel waveguide array.

According to a thirty-second aspect of the present invention, there isprovided an optical communication system comprising: an opticaltransmission means for sending out light signals of differentwavelengths as parallel signals; a multiplexer constituted by an arraywaveguide grating for wavelength multiplexing/demultiplexing each of thedifferent wavelength light signals sent out from the opticaltransmission means; an optical transmission line, to which thewavelength divided and multiplexed light signals outputted from themultiplexer are sent; a node provided in the optical transmission lineand having an array waveguide grating; a demultiplexer constituted by anarray waveguide array for receiving input light signal set along theoptical transmission line via the node; and an optical receiving meansfor receiving the demultiplexed different wavelength light signals fromthe demultiplexer; wherein the demultiplexer includes a predeterminedsubstrate, a first and a second channel waveguide for light wavetransfer on the substrate, a channel waveguide array having a pluralityof component waveguides having lengths progressively increasing with apredetermined difference between adjacent ones of the waveguides on thesubstrate, a first slab waveguide for connecting the ends of the firstchannel waveguides and one end of the channel waveguide array via awaveguide part having a first shape on the substrate, and a second slabwaveguide for connecting one end of the second channel waveguides andthe other end of the channel waveguide array via a waveguide part havinga second shape on the substrate, and at least a part of at least theopen part of each of the first channel waveguides on the side of thefirst slab waveguide or the open part of each of the second channelwaveguides on the side of the second slab waveguide is flaring in anexponential function shape toward the channel waveguide array.

According to a thirty-third aspect of the present invention, there isprovided an optical communication system comprising: an opticaltransmission means for sending out light signals of differentwavelengths as parallel signals; a multiplexer constituted by an arraywaveguide grating for wavelength multiplexing/demultiplexing each of thedifferent wavelength light signals sent out from the opticaltransmission means; an optical transmission line, to which thewavelength divided and multiplexed light signals outputted from themultiplexer are sent; a node provided in the optical transmission lineand having an array waveguide grating; a demultiplexer constituted by anarray waveguide array for receiving input light signal set along theoptical transmission line via the node; and an optical receiving meansfor receiving the demultiplexed different wavelength light signals fromthe demultiplexer; wherein the demultiplexer includes a predeterminedsubstrate, a first and a second channel waveguide for light wavetransfer on the substrate, a channel waveguide array having a pluralityof component waveguides having lengths progressively increasing with apredetermined difference between adjacent ones of the waveguides on thesubstrate, a first slab waveguide for connecting the ends of the firstchannel waveguides and one end of the channel waveguide array via awaveguide part having a first shape on the substrate, and a second slabwaveguide for connecting one end of the second channel waveguides andthe other end of the channel waveguide array via a waveguide part havinga second shape on the substrate, and includes at least the open part ofeach of the first channel waveguides on the side of the first slabwaveguide or the open part of each of the second channel waveguides withrespect to the second slab waveguide has a shape part flaring in anexponential function shape represented by a function of a degree higherthan the second degree toward the channel waveguide array.

According to a thirty-fourth aspect of the present invention, there isprovided an optical communication system comprising: an opticaltransmission means for sending out light signals of differentwavelengths as parallel signals; a multiplexer constituted by an arraywaveguide grating for wavelength multiplexing/demultiplexing each of thedifferent wavelength light signals sent out from the opticaltransmission means; an optical transmission line, to which thewavelength divided and multiplexed light signals outputted from themultiplexer are sent; a node provided in the optical transmission lineand having an array waveguide grating; a demultiplexer constituted by anarray waveguide array for receiving input light signal set along theoptical transmission line via the node; and an optical receiving meansfor receiving the demultiplexed different wavelength light signals fromthe demultiplexer; wherein the demultiplexer including first and secondchannel waveguides for light wave transfer, a channel waveguide arrayhaving a plurality of component waveguides having lengths progressivelyincreasing with a predetermined difference between adjacent ones of thewaveguides, a first slab waveguide disposed between the first channelwaveguides and one end of the channel waveguide array, and a second slabwaveguide disposed between the second channel waveguides and the otherend of the channel waveguide array, and at least the open part of eachof the first channel waveguides on the side of the first slab waveguideor the open part of each of the second channel waveguides on the side ofthe second slab waveguide has an open end with an opening width greaterthan the waveguide width of the first or second channel waveguides, andthe shape directed from the stem part of the open part toward the openend is found on the inner side of rectangular shape of the opening widthand on the outer side of a second degree curve connecting the stem partand the open end.

In a thirty-fifth aspect of the present invention according to thethirty-third aspect, the flaring shape part represented by the functionof a degree higher than the second degree has such a convex shape thatwhen frequency multiplexed Gaussian waveform light waves pass throughtheir waveguides, their characteristics line in a rage between boundaryranges of characteristics with respect to the transmission width and thecross-talk when they pass through the rectangular waveguides and seconddegree function shape waveguides.

According to a thirty-sixth aspect of the present invention, there isprovided an optical communication system comprising a plurality of nodesconnected by transfer lines into a loop form, wavelength multiplexed anddemultiplexed light signals being transferred along the loop formtransfer line, the nodes each including a first array waveguide gratingfor demultiplexing the multiplexed light signal into light signals ofdifferent wavelengths and a second array waveguide grating formultiplexing the demultiplexed light signals of the differentwavelengths, wherein the first array waveguide grating includes apredetermined substrate, a first and a second channel waveguide forlight wave transfer on the substrate, a channel waveguide array having aplurality of component waveguides having lengths progressivelyincreasing with a predetermined difference between adjacent ones of thewaveguides on the substrate, a first slab waveguide for connecting theends of the first channel waveguides and one end of the channelwaveguide array via a waveguide part having a first shape on thesubstrate, and a second slab waveguide for connecting one end of thesecond channel waveguides and the other end of the channel waveguidearray via a waveguide part having a second shape on the substrate, andat least the open part of each of the first channel waveguides on theside of the first slab waveguide or the open part of each of the secondchannel waveguides on the side of the second slab waveguide is flaringin an exponential function shape toward the channel waveguide array.

According to a thirty-seventh aspect of the present invention, there isprovided an optical communication system comprising a plurality of nodesconnected by transfer lines into a loop form, wavelength multiplexed anddemultiplexed light signals being transferred along the loop formtransfer line, the nodes each including a first array waveguide gratingfor demultiplexing the multiplexed light signal into light signals ofdifferent wavelengths and a second array waveguide grating formultiplexing the demultiplexed light signals of the differentwavelengths, wherein the first array waveguide grating includes apredetermined substrate, a first and a second channel waveguide forlight wave transfer on the substrate, a channel waveguide array having aplurality of component waveguides having lengths progressivelyincreasing with a predetermined difference between adjacent ones of thewaveguides on the substrate, a first slab waveguide for connecting theends of the first channel waveguides and one end of the channelwaveguide array via a waveguide part having a first shape on thesubstrate, a second slab waveguide for connecting one end of the secondchannel waveguides and the other end of the channel waveguide array viaa waveguide part having a second shape on the substrate, and at least apart of at least the open part of each of the first channel waveguideson the side of the first slab waveguide or the open part of each of thesecond channel waveguides on the side of the second slab waveguide isflaring in an exponential function shape toward the channel waveguidearray.

According to a thirty-eighth aspect of the present invention, there isprovided an optical communication system comprising a plurality of nodesconnected by transfer lines into a loop form, wavelength multiplexed anddemultiplexed light signals being transferred along the loop formtransfer line, the nodes each including a first array waveguide gratingfor demultiplexing the multiplexed light signal into light signals ofdifferent wavelengths and a second array waveguide grating formultiplexing the demultiplexed light signals of the differentwavelengths, wherein the first array waveguide grating includes apredetermined substrate, a first and a second channel waveguide forlight wave transfer on the substrate, a channel waveguide array having aplurality of component waveguides having lengths progressivelyincreasing with a predetermined difference between adjacent ones of thewaveguides on the substrate, a first slab waveguide for connecting theends of the first channel waveguides and one end of the channelwaveguide array via a waveguide part having a first shape on thesubstrate, a second slab waveguide for connecting one end of the secondchannel waveguides and the other end of the channel waveguide array viaa waveguide part having a second shape on the substrate, and at leastthe open part of each of the first channel waveguides on the side of thefirst slab waveguide or the open part of each of the second channelwaveguides with respect to the second slab waveguide has a shape partflaring in an exponential function shape represented by a function of adegree higher than the second degree toward the channel waveguide array.

According to a thirty-ninth aspect of the present invention, there isprovided an optical communication system comprising a plurality of nodesconnected by transfer lines into a loop form, wavelength multiplexed anddemultiplexed light signals being transferred along the loop formtransfer line, the nodes each including a first array waveguide gratingfor demultiplexing the multiplexed light signal into light signals ofdifferent wavelengths and a second array waveguide grating formultiplexing the demultiplexed light signals of the differentwavelengths, wherein the first array waveguide grating includes firstand second channel waveguides for light wave transfer, a channelwaveguide array having a plurality of component waveguides havinglengths progressively increasing with a predetermined difference betweenadjacent ones of the waveguides, a first slab waveguide disposed betweenthe first channel waveguides and one end of the channel waveguide array,a second slab waveguide disposed between the second channel waveguidesand the other end of the channel waveguide array, at least the open partof each of the first channel waveguides on the side of the first slabwaveguide or the open part of each of the second channel waveguides onthe side of the second slab waveguide has an open end with an openingwidth greater than the waveguide width of the first or second channelwaveguides, and the shape directed from the stem part of the open parttoward the open end is found on the inner side of rectangular shape ofthe opening width and on the outer side of a second degree curveconnecting the stem part and the open end.

In a fortieth aspect of the present invention according to thethirty-eighth aspect, the flaring shape part represented by the functionof a degree higher than the second degree has such a convex shape thatwhen frequency multiplexed Gaussian waveform light waves pass throughtheir waveguides, their characteristics line in a rage between boundaryranges of characteristics with respect to the transmission width and thecross-talk when they pass through the rectangular waveguides and seconddegree function shape waveguides.

Other objects and features will be clarified from the followingdescription with reference to attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first example of the array waveguide grating according tothe first aspect of the present invention;

FIG. 2 shows a first example of the array waveguide grating according tothe second aspect of the present invention;

FIG. 3 shows a first example of the array waveguide grating according tothe third aspect of the present invention;

FIG. 4 shows a first example of the array waveguide grating according tothe fourth aspect of the present invention;

FIG. 5 shows an optical waveguide with exponential shape according tothe first aspect of the present invention;

FIG. 6 shows a first example of the optical waveguide according to thesecond aspect of the present invention;

FIG. 7 shows a second example of the optical waveguide according to thesecond aspect of the present invention;

FIG. 8 shows a third example of the optical waveguide according to thesecond aspect of the present invention;

FIG. 9 shows a fourth example of the optical waveguide according to thesecond aspect of the present invention;

FIG. 10 shows a fifth example of the optical waveguide according to thesecond aspect of the present invention;

FIG. 11 shows a sixth example of the optical waveguide according to thesecond aspect of the present invention;

FIG. 12 shows a seventh example of the optical waveguide according tothe second aspect of the present invention;

FIG. 13 shows an eighth example of the optical waveguide according tothe second aspect of the present invention;

FIG. 14 shows an example of array waveguide grating according to thefifth aspect of the present invention;

FIG. 15 shows a connecting part of the array waveguide grating accordingto the sixth aspect of the present invention;

FIG. 16 shows a connecting part of the array waveguide grating accordingto the seventh aspect of the present invention;

FIG. 17 shows a connecting part of the array waveguide grating accordingto the eighth aspect of the present invention;

FIG. 18 shows a connecting part of the array waveguide grating accordingto the ninth aspect of the present invention;

FIG. 19 shows a connecting part of the array waveguide grating accordingto the tenth aspect of the present invention;

FIG. 20 shows a connecting part of the array waveguide grating accordingto the fifteenth aspect of the present invention;

FIG. 21 shows the construction of a first embodiment of the arraywaveguide grating according to the present invention;

FIG. 22 shows the core shapes of the first channel waveguide and thefirst sector-shape slab waveguide in the boundary part shown in FIG. 21;

FIG. 23 shows the core shapes of the second channel waveguide and thesecond sector-shape slab waveguide in the boundary part shown in FIG.21;

FIG. 24 is an enlarged-scale showing of an exponential function shapeoptical waveguide shown in FIG. 22;

FIG. 25 shows changes in the cross-talk, loss and transmission widthobtained by increasing the value of the shape variable a of theexponential function shape optical waveguides shown in FIG. 22;

FIG. 26 shows changes in the shape of the exponential function shapewaveguide shown in FIG. 24 with changes in the value of the shapevariable a in the equation (1);

FIG. 27 conceptionally shows the difference between the characteristicsof the exponential function shape optical waveguide according to thepresent invention and the prior art second degree function shape opticalwaveguide;

FIG. 28 is a drawing of compared result of an array waveguide gratingusing exponential function shape optical waveguides according to thepresent invention and a prior art array waveguide grating using seconddegree function shape optical waveguides in the spectral shape and thetransmission width;

FIG. 29 shows the outline of the structure of an array waveguide moduleusing the first embodiment of the array waveguide grating as a secondembodiment of the present invention;

FIG. 30 shows the metal plate with the temperature sensor buriedtherein;

FIG. 31 shows the outline of the structure of an optical communicationsystem as a third embodiment of the present invention;

FIG. 32 shows an outline of the construction of the nodes;

FIG. 33 shows an example of prior art array waveguide grating;

FIG. 34 shows, to an enlarged scale, a boundary part between the firstchannel waveguides and the first sector-shape slab waveguide in thearray waveguide grating shown in FIG. 33;

FIG. 35 shows a boundary part in the case of using parabolic or seconddegree function shape waveguides shown in FIG. 33;

FIG. 36 shows an ideal characteristic of wavelength multiplexed lightsignals;

FIG. 37 shows a summary of proposal of an array waveguide grating with arectangular optical waveguide connected to a slab waveguide;

FIG. 38 shows a way of output of wavelength-separated signal afterspread through the channel waveguide and rectangular optical waveguide;

FIG. 39 shows optical frequency characteristics of light signals takenout in the example shown in FIG. 38; and

FIG. 40 shows light signals of two adjacent channels;

PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the present invention will now be describedwith reference to the drawings.

The array waveguide grating according to the first aspect of the presentinvention, comprises the first slab waveguide connecting one end of thefirst channel waveguides and one end of channel waveguide array via thefirst shape waveguide part and the second slab waveguide connecting oneend of the second channel waveguides and the other end of the channelwaveguide array via the second shape waveguide part. The first channelwaveguides may be for the input or the output. As a further alternative,a plurality of waveguides at the same locality may be partly for theinput, while the remainder is for the output. Where the first channelwaveguides are sole channel waveguides for the input, the second channelwaveguides serve for the output. Where the first channel waveguide arefor the output, the remainder is for the input.

In this aspect, at least the open part of each of the first channelwaveguides on the side of the first slab waveguide or the open part ofeach of the second channel waveguides on the side of the second slabwaveguide flares in an exponential function shape toward the channelwaveguide array. In other words, both or either one of the first andsecond shape waveguide parts may have the shape flaring in theexponential function shape. Either the first or the second shapewaveguide part may consist of a plurality of waveguide parts incorrespondence to the channel of each channel waveguide. In the case ofthe present invention of the first aspect, it is not covered that onlypart of such waveguide part connected to a slab waveguide has a shapepart flaring in the exponential function shape. Several examples of thisembodiment will be described in the following.

FIG. 1 shows a first example of the relation between first boundary part108 corresponding to the first boundary part 18 shown in FIG. 33 and asecond boundary part 109 corresponding to the second boundary part 19. Afirst and a second sector-shape slab waveguide 105 and 106 constitutingthe boundary parts 108 and 109 are connected via a channel waveguidearray 104 to each other. In this example, a first to a third exponentialfunction shape optical waveguide 111 ₁ to 111 ₃ having an exponentialfunction shape are provided in the connecting parts between the first tothird first channel waveguides 102 ₁ to 102 ₃ and the first sector-shapeslab waveguide 105. On the other hand, a first to a third taper shapeoptical waveguide 112 ₁ to 112 ₃ are disposed between the first to thirdsecond channel waveguides 103 ₁ to 103 ₃ and the second sector-shapeslab waveguide 106.

FIG. 2 shows a second relation example corresponding to the firstaspect. In this example, the first to third exponential function shapeoptical waveguides 111 ₁ to 111 ₃ and the first to third taper shapeoptical waveguides 112 ₁ to 112 ₃ are disposed conversely to those inthe first example. More specifically, the first to third taper shapeoptical waveguides 112 ₁ to 112 ₃ constituting the first boundary part108 are disposed in the connecting part between the first to thirdchannel waveguides 102 ₁ to 102 ₃ and the first sector-shape slabwaveguide 105, while the first to third exponential function shapeoptical waveguides 111 ₁ to 111 ₃ are disposed in the connecting partsbetween the first to third second channel waveguides 103 ₁ to 103 ₃constituting the second boundary part 109 and the second sector-shapeslab waveguide 106.

FIG. 3 shows a third relation example corresponding to the first aspect.This example is the same in structure as the first example shown in FIG.1. In this example, a first to a third second degree function shapeoptical waveguide 113 ₁ to 113 ₃ are disposed in lieu of the first tothird taper shape light waveguides 112 ₁ to 112 ₃ in the second boundarypart 109. As a converse structure to the third example, it is of coursepossible to dispose the first to third second degree function shapeoptical waveguides 113 ₁ to 113 ₃ in the first boundary part 108 anddispose the first to third exponential function shape optical waveguides111 ₁ to 111 ₃ in the second boundary part 109.

FIG. 4 shows a fourth relation example corresponding to the firstaspect. In this example the first boundary part 108 is the same instructure as that in the first example shown in FIG. 1. In the secondboundary part 109, on the other hand, the first to third second channelwaveguides 103 ₁ to 103 ₃ directly terminate in the second sector-shapeslab waveguide 106. This arrangement can be considered such that thefirst to third taper shape optical waveguides 112 ₁ to 112 ₃ shown FIG.1 have an inclination angle of 0 degree with respect to the secondchannel waveguides 103 ₁ to 103 ₃. As a converse structure to the fourthexample, it is of course possible that first to third first channelwaveguides 102 ₁ to 102 ₃ in the first boundary part 108 directlyterminate in the first sector-shape slab waveguide 105 while disposingthe first to third exponential function shape optical waveguides 111 ₁to 111 ₃ in the second boundary part 109.

Unlike the above arrangement, both the first and second shape waveguideparts may have the shape portion flaring in the exponential functionshape toward the channel waveguide array. This example will be describedin connection with the fifth aspect. The difference of the opticalwaveguide having the exponential function shape such as the first tothird and so forth exponential function shape optical waveguides 111 ₁to 111 ₃ from the rectangular or second degree function shape opticalwaveguide will be described later in detail.

The second aspect is different from the first aspect in that at leastpart of the open part of each first channel waveguide on the side of thefirst slab waveguide and the open part of each second channel waveguideon the side of the second slab waveguide has a shape part flaring in anexponential function shape toward the channel waveguide array. That is,according to the second aspect the scope of application of theexponential function shape is broader than according to the firstaspect. In the first place, where a plurality of waveguide parts areconnected to a slab waveguide, only some of the waveguide parts may havethe exponential function shape flaring shape part. This means that awaveguide part without having any exponential function shape flaringshape part may be present. Where a waveguide part of a channel waveguidecorresponding to a particular channel may have the exponential functionshape flaring shape part, it is not forbidden that this waveguide parthas any other shape part. In other words, a certain waveguide mayconsist of a sole exponential function shape flaring part or of acombination of an exponential function shape flaring shape part and anyother shape part. Also, where a waveguide part consists of a combinationof a plurality of different shape parts, the total length of thewaveguide part may be increased in dependence on the shape combination.As a further alternative, a plurality of shape parts may be present suchas to divide a predetermined length. Some examples concerning the shapecombination according to the second aspect will be given hereinunder.

FIG. 5 shows, in the first place, a connection state like that accordingto the first aspect. In this case, a sole exponential function shapeoptical waveguide 111 is connected between first channel waveguide 102and first sector-shape slab waveguide 105. The state of connectionbetween second channel waveguide 103 and second sector-shape slabwaveguide 106 (see FIG. 1) is alike, and its description is thus notgiven. The same omission of description is done in the following.

FIG. 6 shows a first example of the connection state corresponding tothe second aspect. In this example, a taper shape optical waveguide 112and an exponential function shape optical waveguide 111 are inserted inthe mentioned order between first channel waveguide 102 and firstsector-shape slab waveguide 105.

FIG. 7 shows a second connecting part example corresponding to thesecond aspect. In this example, an exponential function shape opticalwaveguide 111 and a taper shape optical waveguide 112 are inserted inthe mentioned order between first channel waveguide 102 and firstsector-shape slab waveguide 105.

FIG. 8 shows a third connecting part example corresponding to the secondaspect. In this example, a second degree function shape opticalwaveguide 113 and an exponential function shape optical waveguide 111are inserted in the mentioned order between first channel waveguide 102and first sector-shape slab waveguide 105. Although not shown, otherstructures such as a converse structure that the exponential functionshape optical waveguide 111 and the second degree function shape opticalwaveguide 113 are inserted in the mentioned order between the firstchannel waveguide 102 and the first sector-shape slab waveguide 105, areof course conversed by the second aspect.

FIGS. 9 to 13 show further connecting part examples, in which, unlikethe above examples, a waveguide part of a first or a second other shapethan the exponential function shape is provided before or after anexponential function shape optical waveguide. In these examples, thetotal length of the waveguide parts varies in dependence on thecombination status. In the following description, it is assumed that thelength L2 of the exponential function shape optical waveguide 111 shownin FIG. 5 is present within a rectangle determined by width Wp andlength L2.

FIG. 9 shows a fourth connecting part example corresponding to thesecond aspect. In this case, a taper shape optical waveguide 112 withlength L1 is present in front of the exponential function shapewaveguide 111. More specifically, the taper shape optical waveguide 112and the exponential function shape optical waveguide 111 are disposed inthe mentioned order between, for instance, first channel waveguide 102and first sector-shape slab waveguide 105. The total length of theconnecting part is substantially equal to the sum of L1 and L2.Stringently, the length of the exponential function shape lightwaveguide 111 is slightly smaller than L2 because the end of theexponential function shape optical waveguide 111 is in contact with aslightly open part of the taper shape optical waveguide 112. In theFigure, reference symbol Wc designates a core width of the first channelwaveguide 102, and symbol Wp designates an end width.

FIG. 10 shows a fifth connecting part example corresponding to thesecond aspect. In this case, a taper shape optical waveguide with aninclination angle of 0 degree is connected to the trailing end of theexponential function shape optical waveguide. More specifically, theexponential function shape optical waveguide 111 and the taper shapelight waveguide 112 with inclination angle of 0 degree are inserted inthe mentioned order between, for instance, first channel waveguide 102and first sector-shape slab waveguide 105. The total length of thisconnecting part is equal to the sum of L2 and L3.

FIG. 11 shows a sixth connecting part example corresponding to thesecond aspect. This example is the same as the FIG. 10 example exceptfor that a taper shape optical waveguide with an inclination angle otherthan 0 degree is inserted. Again in this example, the total length ofthe connecting part is equal to the sum of L2 and L3.

FIG. 12 shows a seventh connecting part example corresponding to thesecond aspect. In this example, waveguide parts of other shapes than theexponential function shape are inserted before and after the exponentialfunction shape optical waveguide. More specifically, as shown in FIG. 9the taper shape light waveguide 112 is inserted between the firstchannel waveguide 102 and the exponential function shape opticalwaveguide 111, and also a second degree function shape optical waveguide113 is inserted between the exponential function shape optical waveguide111 and the first sector-shape slab waveguide 105. Thus, the totallength of the connecting part is substantially equal to the sum of thelength L1 of the taper shape optical waveguide 112, the intrinsic lengthL2 of the exponential function optical waveguide 111 and the length L3of the second degree function shape optical waveguide 113.

FIG. 13 is an eighth connecting part example corresponding to the secondaspect. In this example, waveguide parts of other shapes than theexponential function shape optical waveguide are continuously inserted.More specifically, a taper shape optical waveguide 112 with length L3and a second channel waveguide 103 with length L4 are inserted in thementioned order between the exponential function shape optical waveguide111 and the first sector-shape slab waveguide 105. Of course, it ispossible to produce waveguide parts having desired characteristics bymany other combinations than those shown before in connection with FIGS.9 to 13.

According to the third aspect, a shape W(X) flaring in exponentialfunction shape, which is adapted in the array waveguide gratingaccording to the first or second aspect is prescribed. As will bedescribed later, it is possible to preset desired optical frequencycharacteristics by selecting an appropriate value of shape variable a.

According to the fourth aspect, by setting the shape variable a to beunity or below it is possible to make clearer he features of theexponential function shape. This will be described later. In the case ofsetting the shape variable to a value greater than unity, meritsobtainable according to the invention can be produced although theextents of the merits vary. According to the third aspect, the values ofthe shape variable a thus include those greater than unity as well.

According to the fifth aspect, it is shown that it is possible to departfrom the values of the shape variable a of the shape variable adoptedfor the first or second shape waveguide part in an array waveguidegrating.

FIG. 14 shows an example of array waveguide grating corresponding to thefifth aspect. In this example, exponential function shape opticalwaveguides used in the two boundary parts are different in shapevariable a. In FIG. 14, parts like those in FIG. 1 are designated bylike reference numerals and symbols, and their detailed description isappropriately omitted. In the boundary part 108, the first to thirdfirst channel waveguides 102 ₁ to 102 ₃ are connected via the first tothird exponential function shape optical waveguides 111A1 to 111A₃ tothe first sector-shape slab waveguide 105. In the boundary part 9, thefirst to third second channel waveguides 103 ₁ to 103 ₃ are connected tothe first to third exponential function shape optical waveguides 111B₁to 111B₃ to the second sector-shape slab waveguide 106. The first tothird exponential function shape optical waveguides 111A₁ to 111A₃ andthe first to third exponential function shape optical waveguides 111B₁to 111B₃ are different in the shape variable 1 from one another.

In the illustrated example, the shape variable a has a greater value inthe boundary part 109 than in the boundary part 108. By providing thedifference in the shape variable a between the two boundary parts 108and 109, it is possible to use exponential function shape opticalwaveguides for both the boundary parts. As a converse example, the shapevariable a may have a greater value in the boundary part 108.

According to the sixth aspect, it is shown that where a plurality ofwaveguide parts each having a shape part flaring in an exponential shapefrom a channel waveguide toward the channel waveguide array areconnected to a sector-shape slab array, the shape variable a of thesewaveguide parts may be preset independently to adequate valuescorresponding to these channel waveguides.

FIG. 15 shows an example of connecting part corresponding to the sixthaspect. In this example of connecting part 108, the first channelwaveguide 102 ₁ is connected via the first exponential function shapeoptical waveguide 111B₁ to the first sector-shape slab waveguide 105.The second first channel waveguide 102 ₂ is connected via the secondexponential function shape optical waveguide 111A₂ to the firstsector-shape slab waveguide 105. The third first channel waveguide 102 ₃is connected via the third exponential function shape optical waveguide111B₃ to the first sector-shape slab waveguide 105. In the firstexponential function shape optical waveguide 111B₁, as described beforein connection with FIG. 14, the shape variable a is greater than in thesecond exponential function shape optical waveguide 111A₁. Also, in thethird exponential function shape optical waveguide 111B₃ connected tothe third first channel waveguide 102 ₃, the shape variable a is greaterthan in the second exponential function shape optical waveguide 111A₁.

As shown, it is possible to set the value of the shape variable aindependently according to the characteristics of the channel waveguides102 or 103. While in the FIG. 15 example two different values of shapevariable a are set, it is also possible to set a maximum number ofdifferent value of shape variable a equal to the number of exponentialfunction shape optical waveguides 111.

According to the seventh aspect, the taper shape is provided as anexample of the shape of a part in the first and second shape waveguideparts, which does not have any shape part flaring in the exponentialfunction shape.

FIG. 16 shows an example corresponding to the seventh aspect. In theboundary part 108, for instance, the second first channel waveguide 102is connected via the second exponential function shape optical waveguide111B₂ to the first sector-shape slab waveguide 105. The remaining firstand third channel waveguides 102 ₁ and 102 ₃ are connected via the firstand third tape shape optical waveguides 112 ₁ and 112 ₃ to the firstsector-shape slab waveguide 105.

According to the eighth aspect, the second degree function shape isprovided as an example of the shape of a part in the first and secondshape waveguide parts, which does not have any shape part faring in theexponential function shape.

FIG. 17 shows an example corresponding to the eighth aspect. The secondexponential function shape optical waveguide 111 ₂ having exponentialfunction shape and the second degree function shape optical waveguides113 ₁ and 113 ₃ are provided on the opposite sides of the waveguide 111₁ in the connecting parts between the first to third first channelwaveguides 102 ₁ to 102 ₃ and the first sector-shape slab waveguide 105.Various modifications of this example such as the converse arrangementthereto are possible.

FIG. 18 shows an example corresponding to the ninth aspect. The secondexponential function shape optical waveguide 111 ₂ is provided in theconnecting part between the second first channel waveguide 102 ₂ and thefirst sector-shaped slab waveguide 105. The first taper shape opticalwaveguide 112 ₁ is provided in the uppermost connecting part in theFigure between the first channel waveguide 102 ₁ and the firstsector-shape slab waveguide 105. The third second degree function shapeoptical waveguide 113 ₃ is provided in the lowermost connecting part inthe Figure between the third first channel waveguide 102 ₃ and the firstsector-shape slab waveguide 105.

While FIG. 18 shows the case where a total of three channel waveguides,i.e., the first to third first channel waveguides 102 ₁ to 102 ₃ arepresent, in the case where a greater number of channel waveguides arepresent, it is possible to select, as desired, the number of opticalwaveguides, which are each constituted as taper shape optical waveguides112 as part without any part flaring in an exponential function shape,and also the number of optical waveguides, which are each constituted assecond degree function shape optical waveguide 113 as such part. Ofcourse, these arrangements may concern not only the boundary part 108but also the boundary part 109. Also, one of the optical waveguides maybe constituted by the taper shape optical waveguide 112 and the seconddegree function shape optical waveguide 113 inserted in the mentionedorder.

According to the tenth aspect, as shown in FIG. 7, an example ofwaveguide is provided, in which exponential function shape opticalwaveguide 111 and taper shape optical waveguide 112 are inserted in thementioned order between each first channel waveguide 102 and firstsector-shape slab waveguide 105. The numbers and arrangement order ofexponential function shape optical waveguides 111 and taper shapeoptical waveguides 112 can be selected as desired.

FIG. 19 shows an example corresponding to the tenth aspect. The secondexponential function shape optical wave guide 111 ₂ having a soleexponential function shape is provided in the connecting part betweenthe second first channel waveguide 102 ₁ and the first sector-shape slabwaveguide 105. The first exponential function shape optical waveguide111 ₁ and the first taper shape optical waveguide 112 ₁ are inserted inthe mentioned order in the uppermost connecting part in the Figurebetween first first channel waveguide 102 ₁ and the first sector-shapeslab waveguide 105. The third exponential function shape opticalwaveguide 111 ₃ and the third taper shape optical waveguide 112 ₃ areinserted in the mentioned order in the lowermost connecting part betweenthe third exponential function shape optical waveguide 111 ₃ and thethird taper shape optical waveguide 112 ₃. Various variations in theinsertion order and so forth are of course possible.

According to the eleventh aspect, as shown in FIG. 8, an example ofwaveguide is provided, in which second degree function shape opticalwaveguide 113 and exponential function shape optical waveguide 111 areinserted in the mentioned order between each first channel waveguide 102and the first sector-shape slab waveguide 105. The numbers andarrangement order of the exponential function shape optical waveguides111 and second degree function shape optical waveguides 113 can beselected as desired.

According to the twelfth aspect, it is shown that an optical waveguidehaving an exponential function shape flaring shape part may have, asother shape part, not only taper shape part but also second degreefunction shape part.

FIG. 20 shows an example corresponding to the twelfth aspect. The secondexponential function shape optical waveguide 111 ₂ having a soleexponential function shape is provided in the connecting part betweenthe second first channel waveguide 102 ₂ and the first sector-shape slabwaveguide 105. The first taper shape optical waveguide 112 ₁ and thefirst exponential function shape optical waveguide 111 ₁ are inserted inthe mentioned order in the uppermost connecting part in the Figurebetween the first first channel waveguide 102 ₁ and the firstsector-shape slab waveguide 105. The third second degree function shapeoptical waveguide 113 ₃ and the third exponential function shape opticalwaveguide 111 ₃ are inserted in the mentioned order in the lowermostconnecting part in the Figure between the third first channel waveguide102 ₃ and the first sector-shape slab waveguide 105. Various variationsin the insertion order and so forth are of course possible.

According to the thirteenth aspect, at least either the open part ofeach of the first waveguides on the side of the first slab waveguides orthe open part of each of the second channel waveguides on the side ofthe second slab waveguide is a shape part flaring in a shape representedby a function of the second or a higher degree. As will be describedlater, in this case, again in this case the same feature as isobtainable in the case of the exponential function shape flaring shapepart can be obtained. Also, again in the case of the thirteenth aspect,various variations concerning combinations of other shapes and so forthare possible. However, these variations are essentially like those inthe case involving the exponential function shape flaring shape part,and they are not exemplified.

According to the fourteenth aspect, at least either the open part ofeach of the first channel waveguides on the side of the first slabwaveguide or the open part of each of the second channel waveguides onthe side of the second slab waveguide has an open end with an openingwidth greater than the waveguide width of the first or second channelwaveguides, and has a curved shape such that its part extending from hestem part toward the open end is on the inner side of rectangular shapeof the opening width and on the outer side of a second degree curveconnecting the stem part and the open end.

According to the fifteenth aspect, it is featured that in the arraywaveguide grating as the thirteenth aspect, each flaring shape portionrepresented by a second or a higher degree function has a convex shapehaving a characteristic in a region between the boundary regions of thecharacteristics with respect to the transmission width and thecross-talk as each light wave of multiplexed Gaussian waveform passingthrough the waveguide thereof passes rectangular waveguide and seconddegree function shape waveguide. This will be described hereinafter withreference to drawings.

According to the sixteenth aspect, it is shown that array waveguidegrating modules in the form of combinations of optical fibers with thearray waveguide gratings according to the first aspect.

In the other aspects such as twenty-first to twenty-eighth aspects,FIGS. 14 to 20 should be referred. Other aspects will be understood fromthe foregoing and following description of the present invention.

Now, preferred embodiments of the present invention will be describedwith reference to the drawings.

FIG. 21 shows the construction of a first embodiment of the arraywaveguide grating according to the present invention. The illustratedarray waveguide grating 100 has a substrate 101, which one or more firstchannel waveguides 102, a plurality of second channel waveguide 103, achannel waveguide array 104 with a plurality of component channelwaveguides bent in a predetermined direction with different radii ofcurvature, a first sector-shape slab waveguide 104 connecting the firstchannel waveguides 102 and the channel waveguide array 104 to oneanother and a second sector-shape slab waveguide 106 connecting thechannel waveguide array 104 and the second channel waveguides 103 to oneanother, are formed on. Multiplexed light signals with wavelengths λ₁ toλ_(n) are incident from the first channel waveguide 102, then proceedwith their paths expanded therethrough and are then incident on thechannel waveguide array 104.

In the channel waveguide array 104, the component array waveguides haveprogressively increasing or reducing optical path lengths with apredetermined optical path length difference provided between adjacentones of them. Thus, the light beams proceeding through the individualarray waveguide reach the second sector-shape slab waveguide 106 with apredetermined phase difference provided between adjacent ones of them.Actually, wavelength dispersion takes place, and in-phase plane isinclined in dependence on the wavelength. Consequently, the light beamsare focused (i.e., converged) on the boundary surface between the secondsector-shape slab waveguide 106 and the plurality of second channelwaveguide 103 at positions different with wavelengths. The secondchannel waveguides 103 are disposed at positions corresponding to theirrespective wavelengths. Given wavelength components λ₁ to λ_(n) thus canbe taken out independently from the second channel waveguides 103.

The structure of this embodiment of the array waveguide grating 100 willnow be described specifically. In this embodiment, a semiconductor(i.e., silicon) substrate is used as the substrate 101. Of course, thesemiconductor is by no means limitative as the material of the substrate101. In this embodiment, the substrate 101 is formed by using aquartz-based material doped with phosphorus, germanium, titanium, boron,fluorine, etc. for a lower clad layer, which is deposited to a thicknessof several ten micrometers by using such method as fire depositionmethod, normal pressure CVD (Chemical Vapor Deposition) method,spattering method, spin coating method and electron beam depositionmethod. On this layer, a core layer having an optical waveguide shape asshown in FIG. 21 is formed.

In the core layer formation, fine areas are transferred to an adequatemask material by using litho-photography. Then, unnecessary areas areremoved by a dry etching method using an RIE (Reactive Ion Etching)device or an RIBE (Reactive Ion Beam Etching) device. Finally, an upperclad layer about several ten micrometers thick is deposited by usingagain the above quartz material with a refractive index preset to behigher than that of the core layer.

FIG. 22 shows the core shapes of the first channel waveguide 102 and thefirst sector-shape slab waveguide 105 in the boundary part 108 shown inFIG. 21. For the sake of the brevity of illustration, it is assumed thatthe first channel waveguide 102 shown in FIG. 21 is constituted by threechannel waveguides, i.e., the first to third first channel waveguides102 ₁ to 102 ₃. The first to third first channel waveguides 102 ₁ to 102₃ are connected via the first to third exponential function shapeoptical waveguides 111 ₁ to 111 ₃ having a corresponding exponentialfunction shape to the first sector-shape slab waveguide 105.

FIG. 23 shows the core shapes of the second channel waveguide 103 andthe second sector-shape slab waveguide 106 in the boundary part 109shown in FIG. 21. Again in this Figure, for the sake of the brevity ofillustration, the second channel waveguide 103 shown in FIG. 21 isconstituted by three channel waveguides, i.e., the first to third secondchannel waveguides 103 ₁ to 103 ₃. The first to third second channelwaveguides 103 ₁ to 103 ₃ are connected via the first to third tapershape optical waveguides 112 ₁ to 112 ₃ having a corresponding tapershape to the second sector-shape slab waveguide 106.

The shape of the cores of the boundary part between the firstsector-shape slab waveguide 105 and the channel waveguide array 104, andbetween the first sector-shape slab waveguide 105 and the secondsector-shape slab waveguide 106 as shown in FIG. 21 is a taper shape asshown in FIG. 23. The shape of these parts, however, have no directbearing on the optical frequency characteristic, and is thus outside thesubject of consideration according to the present invention. Accordingto the present invention, optical frequency characteristics of the firstand second boundary parts 108 and 109, i.e., the optical frequencycharacteristics of the connecting part between the channel waveguide 102and the sector-shape slab waveguide 105 and the connecting part betweenthe channel waveguide 103 and the sector-shape slab waveguide 106 basedon the shapes of the optical waveguides, are considered.

By the way, light beams incident on the first channel waveguides 102 onthe substrate 101 shown in FIG. 21, are changed from basic mode toharmonic mode as it passes through the exponential function shapeoptical waveguides 111. As a result, the electric field distribution isconverted from Gaussian distribution to a specially flat electric fielddistribution. The spread of this electric field distribution isdetermined by an optical waveguide shape, which is represented byoptical waveguide width W(X) given by the following equation (1).W(X)=(Wp−Wc)*(1−exp(−a*X))+Wc  (1)where symbol X represents the progress direction of the light waves,symbol a represents the shape variable giving the shape of theexponential function, and symbol Wc the core width of the channelwaveguide. Symbol Wp represents the end with of connection of theexponential function shape optical waveguide 111 to the firstsector-shape slab waveguide 105 as shown in FIG. 22.

When light beams having such electric field distribution are incident onthe first sector-shape slab waveguide 105 shown in FIG. 21 after passingthrough the exponential function shape optical waveguides 111, theirpaths are expanded in directions perpendicular to the optical axis. Thelight beams then excite the component waveguides of the channelwaveguide array 104, and are converged in the second sector-shape slabwaveguide 103 at positions of the second channel waveguides 103corresponding to their optical frequencies f. In this way, light beamsof desired wavelength components λ₁ to λ_(n) are taken out from theindividual second channel waveguides 103.

The core opening width Wt of the second channel waveguides 103 shown inFIG. 23 are designed to be less than the above spacially flat electricfield distribution width. Thus, the quantity of light coupled to thesecond channel waveguides 103 is substantially fixed irrespective ofslight changes in the optical frequency f of a light source (not shown).It is thus possible to obtain flat optical frequency characteristics,with which the split light beam output is substantially fixedirrespective of changes in the optical frequency f of the light source.

FIG. 24 is an enlarged-scale showing of an exponential function shapeoptical waveguide shown in FIG. 22. The parameters in the previousequation (1) are shown in the Figure. As shown in the Figure, the lengthof the exponential function shape optical waveguide 111 in the directionX of the light wave progress is designated by L2. The dashed line shows,for the sake of reference, a rectangular optical waveguide 114, which isdetermined by width Wp and length L2.

FIG. 25 shows changes in the cross-talk, loss and transmission widthobtained by increasing the value of the shape variable a of theexponential function shape optical waveguides shown in FIG. 22 from0.01. These characteristic curves correspond to a case with a width Wpof 18 μm, a waveguide part spread Wc of 5.5 μm and a length L2 of 175μm. The broader the transmission width is, it is the better, and thegreater the absolute value is, it is the better. Thus, the upperpositions the transmission width curve 201 and the cross-talk absolutevalue curve 202 are in, the results are the better. Excess loss is shownby curve 203.

As described before in connection with the prior art, the transmissionwidth and the cross-talk are in the trade-off relation to each other.More specifically, to increase the transmission width represented by thecurve 201 the value of the shape variable a has to be increased as shownby arrow 204. Also, to increase the cross-talk absolute valuerepresented by the curve 202 the value of the shape variable a has to bereduced as shown by arrow 205. However, both the characteristic curvesundergo only slight changes with increase of the value of the shapevariable a beyond 0.1, and are hardly changed with a value of the shapevariable a greater than unity. Accordingly, the value of the shapevariable a can be limited to be unity or below without any practicaltrouble in considerations.

FIG. 26 shows changes in the shape of the exponential function shapewaveguide shown in FIG. 24 with changes in the value of the shapevariable a in the equation (1). Here, a case with the shape variable aless than unity is considered for the ground described before. Morespecifically, the shape variable a is changed from unity to 0.02. Whenthe shape variable a is unity, it is possible to obtain a result closeto the rectangular shape optical waveguide 114 and thus obtain a maximumtransmission width as shown in FIG. 25. By reducing the shape variablean absolute value of the cross-talk is increased. The closer the valueof the shape variable a is to zero, the closer is the characteristic tothat of the second degree shape optical waveguide 113 (see FIG. 3).

From FIG. 26, it will be seen that with the exponential function shapeoptical waveguide 111 it is possible to obtain a characteristic, whichis superior to that of the second degree function shape opticalwaveguide 113 to an extent that the waveguide 111 is closer incharacteristic to the rectangular shape optical waveguide 114. Also, itwill be seen that the degree of freedom of the characteristic choice ishigh owing to the possibility of varying the value of the shape variablea.

FIG. 27 conceptionally shows the difference between the characteristicsof the exponential function shape optical waveguide according to thepresent invention and the prior art second degree function shape opticalwaveguide. In the Figure, the ordinate is taken for the transmissionwidth, and the abscissa is taken for cross-talk in absolute value. Thegreater the both values are, the state is the better, and the closer thetwo values to the origin (i.e., zero), the state is the worse. Straightline 116 shows an example of the exponential function shape opticalwaveguide, and broken line 117 shows an example of the second degreefunction shape optical waveguide. These characteristics vary independence on the values of the shape variable, and they are specifiedto be within the ranges enclosed in dashed loops 118 and 119. Theexponential function shape optical waveguide becomes infinitely closerto a rectangle as the shape variable a becomes infinity. On the otherhand, the waveguide assumes a linear taper shape when the variable abecomes zero. The exponential function shape optical waveguide also isin contact with the second degree function shape optical waveguide independence on the value of the shape variable. However, as is obviousfrom FIG. 27, outside the part where the two optical waveguides are incontact, the exponential function shape optical waveguide is alwayscharacteristically superior in both the transmission width and theabsolute value of the cross-talk to the second degree function shapeoptical waveguide.

FIG. 28 is a drawing of compared result of an array waveguide gratingusing exponential function shape optical waveguides according to thepresent invention and a prior art array waveguide grating using seconddegree function shape optical waveguides in the spectral shape and thetransmission width. In the Figure, solid plot 211 represents the resultof measurements with the exponential function shape optical waveguides111 shown in, for instance, FIG. 113, and dashed plot 212 represents theresult of measurements with the second degree function shape opticalwaveguides 113 shown in, for instance, FIG. 3. The values of the widthWp and so forth are the same as those shown in FIG. 25.

From the Figure it is obvious that the second degree shape opticalwaveguides 113 shown by the dashed plot 212 has a shaper spectral shapeand also has a smaller transmission width than that of the exponentialfunction shape optical waveguide 111 shown by the solid plot 211. Inthis embodiment, adjacent channels of light signals are present in theneighborhood of 0.8 in the abscissa, and at this position the cross-talkis lower with the second degree function shape optical waveguide 111than with the exponential function shape optical waveguide 113. That is,with the exponential function shape optical waveguide 111 the influenceof the adjacent channel light signals is less. Thus, it will be seenthat the exponential function shape optical waveguide 111 is improved inthe transmission width and the cross-talk over the second degreefunction shape optical waveguide 111.

FIG. 29 shows the outline of the structure of an array waveguide moduleusing the first embodiment of the array waveguide grating describedabove as a second embodiment of the present invention. The illustratedarray waveguide grating module 301 comprises a box-like case 302, atemperature control element 303 disposed on the bottom of the case 302and constituted by a Velch element for heating and cooling, an arraywaveguide grating 100 and a metal plate 305 intervening between thetemperature control element 303 and the array waveguide element 100. Inthis embodiment, the metal plate 305 is a high heat conductivity copperplate. The metal plate 305 has a size greater than the contact size ofthe temperature control element 303 to provide for enlarged temperaturecontrol zone of the temperature control element 303.

The metal plate 305 has a groove, in which a temperature sensor 306 isburied together with high heat conductivity material 307. The detectedtemperature output is inputted to a temperature control circuit 308 fortemperature control of the temperature control element 303. Thetemperature sensor 306 buried in the metal plate 305 is led out to theoutside from a position 309. In this embodiment, the temperature sensor306 is a thermistor.

Optical fibers 311 and 312 are led out from the side of the first andsecond channel waveguides 102 and 103 of the array waveguide grating 100to the outside of the case 302. The optical fiber 311 has one endconnected to the first channel waveguide 102 and the other end connectedto a light source side (not shown). The optical fiber has one endconnected to the second channel waveguide 103 and the other endconnected to a circuit part (not shown) for processing light signalsafter demultiplexing.

FIG. 30 shows the metal plate with the temperature sensor buriedtherein. Shown enclosed in dashed rectangle 321 on the metal plate 305is a temperature detection area, which is in contact with an areaincluding the channel waveguide array 104 and the first and secondsector-shape slab waveguides 105 and 106. By controlling the temperatureof the area 321 to a predetermined temperature by highly accuratelydetecting the temperature, it is possible to prevent characteristicschanges due to temperature variations in the array waveguide grating100.

The surface of the metal plate 305 has a groove 22, which is cut such asto extend from the substantial center of the temperature detection areaand draw an angularly spiral trace. In the groove 22, the temperaturesensor 306 is buried together with the high heat conductivity material307. The temperature sensor 306 has a temperature sensing part 323located at its end, which is buried in the temperature detecting area321 substantially at the center thereof. From this position, a pair ofled lines 325 and 326 are led spirally through the metal plate 305 andto the outside from position 309. The pair lead lines 325 and 326 arerelatively thin lines.

In this embodiment of the array waveguide grating module 301, thetemperature sensing part 323 of the temperature sensor 306 is buried inthe metal plate 305 and the groove 322 is closed by the array waveguidegrating 100. Also, as shown in FIG. 30, the pair leads 325 and 326 whichare liable to feed back heat to the temperature sensing part 323, areburied together with the high heat conductivity material 307 in themetal plate 305. Furthermore, the lead lines 325 and 326 are not ledfrom the position of the temperature sensing part 323 straight ashortest distance through the metal plate 305, but it is led spirally asone form of curved trace to provide an increased distance.

It will bee seen that, instead of thermal feedback of the ambienttemperature from the position 309 through the lead lines 325 and 326 tothe temperature sensing part 323, heat energy corresponding to thetemperature change is absorbed in the metal plate 305 through therelatively long lead lines 325 and 326 buried therein. Since the metalplate 305 itself is temperature controlled to a predeterminedtemperature by the temperature control element 303, the influence of theambient temperature from the neighborhood of the position 309, like theambient temperature influence from the other part of the metal plate305, is weakened as one goes into the metal plate 305. Thus, in thetemperature detecting area 321 located in the neighborhood of the centerof the inside of the metal plate 105, the lead lines 325 and 326 aresubstantially at the same temperature as the metal in this area. Thus,the ambient temperature can not be considered to be fed back to thetemperature sensing part 323.

Thus, in the array waveguide grating module 301, the temperature sensingpart 323 can accurately measure the temperature of part of the arraywaveguide grating 100 corresponding to the temperature detection area321 without being adversely affected by the ambient temperature, and itis possible to realize stable temperature control irrespective ofambient temperature changes.

FIG. 31 shows the outline of the structure of an optical communicationsystem as a third embodiment of the present invention. In this opticalcommunication system, light signals of N channels of wavelengths λ₁ toλ_(n) sent out from an optical transmitter 401 connected to an SONET(Synchronous Optical Network) apparatus (not shown) disposed on thetransmission side, are multiplexed in an optical MUX (multiplexer) 402,and amplified in a booster amplifier 403 and then sent out to an opticaltransmission line 404. The multiplexed light signal 405 is appropriatelyamplified in an in-line amplifier 406, and then fed through apre-amplifier 407 to an optical demultiplexer (DMUX) 408 fordemultiplexing to the initial wavelengths λ₁ to λ_(n), which arereceived in an optical receiver 409. A suitable number of nodes (ADM)411 ₁ to 411 _(M) are disposed in the optical transmission line 404.Light signals having desired wavelengths are inputted in and outputtedfrom the nodes 411 ₁ to 411 _(M). The optical demultiplexer 408 isconstituted by an array waveguide grating 100 like that as shown in FIG.21.

FIG. 32 shows an outline of the construction of the nodes. Here, thefirst node 111 ₁ (see FIG. 31) is shown. The second to M-th nodes 411 ₂to 411 _(M) are principally the same in structure. In the opticaltransmission line 404 shown in FIG. 21 is inputted to an input sidearray waveguide grating 421 of the first node 4111 for demultiplexing itlight signals of N channels of wavelengths λ1 to λ_(n). Two-inputtwo-output optical switches 422 ₁ to 422 _(N) provided for thewavelengths λ₁ to λ_(n), respectively, drop the light signals of thewavelengths λ₁ to λ_(n) in the node side receiver 426, while transmittedlight signals are added from the node side transmitter 424. The outputsof the two-input two-output optical switches 422 ₁ to 422 _(N) are gainadjusted in ATTs (attenuators) 427 ₁ to 427 _(N) and then inputted to anoutput side array waveguide grating 428. The output side array waveguidegrating 428 is an element having a converse structure to the input sidearray waveguide grating 421. The light signals of the N channels ofwavelengths λ₁ to λ_(n) are multiplexed and sent out to the opticaltransmission line 404 as the light signal 405.

As shown, as well as the first node 411 ₁ shown in FIG. 32, the secondto M-th nodes 411 ₂ to 411 _(M) and the optical demultiplexer 408 alluse the array waveguide grating 100 shown in FIG. 21. The above lightsignal of the wavelength λ_(m) outputted from the output side waveguide(i.e., monitor waveguide) when monitor light signal is inputted from theinput side waveguide, are progressively monitored for wavelengthcompensation of the other output side waveguides, to which the lightsignals of wavelengths λ₁ to λ_(n) are outputted. Thus, as shown in FIG.31 the nodes 411 ₁ to 411 _(M) and the optical receiver 409 have outputmonitor/control units 431 ₁ to 431 _(M) and 431 _(R), respectively.

The array waveguide grating 100, when used as multiplexer, can effectlike wavelength compensation by progressively monitoring the lightsignal of wavelength in outputted from the intrinsic input sidewaveguide (monitor waveguide) with monitor light signal inputted fromthe intrinsic output side waveguide. Although not shown in thisembodiment, it is likewise possible to make compensation of the arraywaveguide grating 101 on the side of the output side array waveguidegrating 428 in the optical transmitter 401 and the nodes 411 ₁ to 411_(M). To this end, output monitor/control units may be provided.

The above embodiments have been described by assuming that they useoptical waveguides including exponential function shape part. Theexponential function, however, is developed as a function of a highdegree such as the third degree. In this sense, the present invention isof course applicable as well to the case, in which waveguides of orhaving shapes represented by high degree functions other than the seconddegree, for instance third or fourth degree, are inserted betweenchannel waveguides and slab waveguide.

As has been described in the foregoing, according to the first, third,twelfth, sixteenth, nineteenth to twenty-eighth, thirty-first andthirty-sixth aspects, at least either the open part of each of the firstchannel waveguides on the side of the first slab waveguide or the openpart of each of the second channel waveguides of the side of the secondslab waveguide has a shape part flaring in the exponential functionshape toward the channel waveguide array, not only it is possible toimprove the optical frequency characteristics of light signals over thecase involving the sole second degree function shape optical waveguideshape but it is also possible to improve the degree of freedom of designover the case involving the sole rectangular or taper shape opticalwaveguide. It is thus possible to cope with various demands flexibly.

Also, according to the second, third to thirteenth, seventeenth,twentieth to twenty-eighth, thirty-second and thirty-seventh aspects, atleast the open part of each of the first channel waveguide on the sideof the first slab waveguide or the open part of each of the secondchannel waveguides on the side of the second slab waveguide at leastpartly has a shape part flaring in the exponential function shape towardthe channel waveguide array, not only it is possible to improve theoptical frequency characteristics of light signals over the caseinvolving the sole second degree function shape optical waveguide shapebut also it is possible improve the degree of freedom of design over thecase involving the sole rectangular or taper shape optical waveguide. Itis thus possible to cope with various demands flexibly.

Furthermore, according to the fifteenth, twenty-ninth, thirty-third andfortieth aspects, at least either the open part of each of the firstchannel waveguides on the side of the first slab waveguide or the openpart of each of the second channel waveguides on the side of the secondslab waveguide has a shape portion flaring in the exponential functionshape represented by a function of a degree higher than the seconddegree, not only it is possible to improve the optical frequencycharacteristics of light signals over the case involving the sole seconddegree function shape optical waveguide shape but it is also possible toimprove the degree of freedom of design over the case involving the solerectangular or taper shape optical waveguide. It is thus possible tocope with various demands flexibly.

According to the fourteenth, eighteenth, twenty-fourth and thirty-firstto fortieth aspects, it is possible to increase the light signaltransmission width over the prior art case involving the second degreefunction shape optical function shape. Thus, in the case of connectingmultiple stages of array waveguide gratings, it is possible to obtainthe advantage that the rate of reduction of the signal transmissionbandwidth is reduced.

Changes in construction will occur to those skilled in the art andvarious apparently different modifications and embodiments may be madewithout departing from the scope of the present invention. The matterset forth in the foregoing description and accompanying drawings isoffered by way of illustration only. It is therefore intended that theforegoing description be regarded as illustrative rather than limiting.

1. An optical communication system comprising: an optical transmissionmeans for sending out light signals of different wavelengths as parallelsignals; a multiplexer constituted by an array waveguide grating forwavelength multiplexing/demultiplexing each of the different wavelengthlight signals sent out from the optical transmission means; an opticaltransmission line, to which the wavelength divided and multiplexed lightsignals outputted from the multiplexer are sent; a node provided in theoptical transmission line and having an array waveguide grating; ademultiplexer constituted by an array waveguide array for receivinginput light signal set along the optical transmission line via the node;and an optical receiving means for receiving the demultiplexed differentwavelength light signals from the demultiplexer; wherein thedemultiplexer includes a predetermined substrate, a first and a secondchannel waveguide for light wave transfer on the substrate, a channelwaveguide array having a plurality of component waveguides havinglengths progressively increasing with a predetermined difference betweenadjacent ones of the waveguides on the substrate, a first slab waveguidefor connecting the ends of the first channel waveguides and one end ofthe channel waveguide array via a waveguide part having a first shape onthe substrate, and a second slab waveguide for connecting one end of thesecond channel waveguides and the other end of the channel waveguidearray via a waveguide part having a second shape on the substrate, andat least the open part of each of the first channel waveguides on theside of the first slab waveguide or the open part of each of the secondchannel waveguides on the side of the second slab waveguide is flaringin an exponential function shape toward the channel waveguide array. 2.An optical communication system comprising: an optical transmissionmeans for sending out light signals of different wavelengths as parallelsignals; a multiplexer constituted by an array waveguide grating forwavelength multiplexing/demultiplexing each of the different wavelengthlight signals sent out from the optical transmission means; an opticaltransmission line, to which the wavelength divided and multiplexed lightsignals outputted from the multiplexer are sent; a node provided in theoptical transmission line and having an array waveguide grating; ademultiplexer constituted by an array waveguide array for receivinginput light signal set along the optical transmission line via the node;and an optical receiving means for receiving the demultiplexed differentwavelength light signals from the demultiplexer; wherein thedemultiplexer includes a predetermined substrate, a first and a secondchannel waveguide for light wave transfer on the substrate, a channelwaveguide array having a plurality of component waveguides havinglengths progressively increasing with a predetermined difference betweenadjacent ones of the waveguides on the substrate, a first slab waveguidefor connecting the ends of the first channel waveguides and one end ofthe channel waveguide array via a waveguide part having a first shape onthe substrate, and a second slab waveguide for connecting one end of thesecond channel waveguides and the other end of the channel waveguidearray via a waveguide part having a second shape on the substrate, andat least a part of at least the open part of each of the first channelwaveguides on the side of the first slab waveguide or the open part ofeach of the second channel waveguides on the side of the second slabwaveguide is flaring in an exponential function shape toward the channelwaveguide array.
 3. An optical communication system comprising: anoptical transmission means for sending out light signals of differentwavelengths as parallel signals; a multiplexer constituted by an arraywaveguide grating for wavelength multiplexing/demultiplexing each of thedifferent wavelength light signals sent out from the opticaltransmission means; an optical transmission line, to which thewavelength divided and multiplexed light signals outputted from themultiplexer are sent; a node provided in the optical transmission lineand having an array waveguide grating; a demultiplexer constituted by anarray waveguide array for receiving input light signal set along theoptical transmission line via the node; and an optical receiving meansfor receiving the demultiplexed different wavelength light signals fromthe demultiplexer; wherein the demultiplexer includes a predeterminedsubstrate, a first and a second channel waveguide for light wavetransfer on the substrate, a channel waveguide array having a pluralityof component waveguides having lengths progressively increasing with apredetermined difference between adjacent ones of the waveguides on thesubstrate, a first slab waveguide for connecting the ends of the firstchannel waveguides and one end of the channel waveguide array via awaveguide part having a first shape on the substrate, and a second slabwaveguide for connecting one end of the second channel waveguides andthe other end of the channel waveguide array via a waveguide part havinga second shape on the substrate, and includes at least the open part ofeach of the first channel waveguides on the side of the first slabwaveguide or the open part of each of the second channel waveguides withrespect to the second slab waveguide has a shape part flaring in anexponential function shape represented by a function of a degree higherthan the second degree toward the channel waveguide array.
 4. The arraywaveguide grating according to claim 3, wherein the flaring shape partrepresented by the function of a degree higher than the second degreehas such a convex shape that when frequency multiplexed Gaussianwaveform light waves pass through their waveguides, theircharacteristics line in a rage between boundary ranges ofcharacteristics with respect to the transmission width and thecross-talk when they pass through the rectangular waveguides and seconddegree function shape waveguides.
 5. An optical communication systemcomprising: an optical transmission means for sending out light signalsof different wavelengths as parallel signals; a multiplexer constitutedby an array waveguide grating for wavelength multiplexing/demultiplexingeach of the different wavelength light signals sent out from the opticaltransmission means; an optical transmission line, to which thewavelength divided and multiplexed light signals outputted from themultiplexer are sent; a node provided in the optical transmission lineand having an array waveguide grating; a demultiplexer constituted by anarray waveguide array for receiving input light signal set along theoptical transmission line via the node; and an optical receiving meansfor receiving the demultiplexed different wavelength light signals fromthe demultiplexer; wherein the demultiplexer including first and secondchannel waveguides for light wave transfer, a channel waveguide arrayhaving a plurality of component waveguides having lengths progressivelyincreasing with a predetermined difference between adjacent ones of thewaveguides, a first slab waveguide disposed between the first channelwaveguides and one end of the channel waveguide array, and a second slabwaveguide disposed between the second channel waveguides and the otherend of the channel waveguide array, and at least the open part of eachof the first channel waveguides on the side of the first slab waveguideor the open part of each of the second channel waveguides on the side ofthe second slab waveguide has an open end with an opening width greaterthan the waveguide width of the first or second channel waveguides, andthe shape directed from the stem part of the open part toward the openend is found on the inner side of rectangular shape of the opening widthand on the outer side of a second degree curve connecting the stem partand the open end.
 6. An optical communication system comprising aplurality of nodes connected by transfer lines into a loop form,wavelength multiplexed and demultiplexed light signals being transferredalong the loop form transfer line, the nodes each including a firstarray waveguide grating for demultiplexing the multiplexed light signalinto light signals of different wavelengths and a second array waveguidegrating for multiplexing the demultiplexed light signals of thedifferent wavelengths, wherein the first array waveguide gratingincludes a predetermined substrate, a first and a second channelwaveguide for light wave transfer on the substrate, a channel waveguidearray having a plurality of component waveguides having lengthsprogressively increasing with a predetermined difference betweenadjacent ones of the waveguides on the substrate, a first slab waveguidefor connecting the ends of the first channel waveguides and one end ofthe channel waveguide array via a waveguide part having a first shape onthe substrate, and a second slab waveguide for connecting one end of thesecond channel waveguides and the other end of the channel waveguidearray via a waveguide part having a second shape on the substrate, andat least the open part of each of the first channel waveguides on theside of the first slab waveguide or the open part of each of the secondchannel waveguides on the side of the second slab waveguide is flaringin an exponential function shape toward the channel waveguide array. 7.An optical communication system comprising a plurality of nodesconnected by transfer lines into a loop form, wavelength multiplexed anddemultiplexed light signals being transferred along the loop formtransfer line, the nodes each including a first array waveguide gratingfor demultiplexing the multiplexed light signal into light signals ofdifferent wavelengths and a second array waveguide grating formultiplexing the demultiplexed light signals of the differentwavelengths, wherein the first array waveguide grating includes apredetermined substrate, a first and a second channel waveguide forlight wave transfer on the substrate, a channel waveguide array having aplurality of component waveguides having lengths progressivelyincreasing with a predetermined difference between adjacent ones of thewaveguides on the substrate, a first slab waveguide for connecting theends of the first channel waveguides and one end of the channelwaveguide array via a waveguide part having a first shape on thesubstrate, a second slab waveguide for connecting one end of the secondchannel waveguides and the other end of the channel waveguide array viaa waveguide part having a second shape on the substrate, and at least apart of at least the open part of each of the first channel waveguideson the side of the first slab waveguide or the open part of each of thesecond channel waveguides on the side of the second slab waveguide isflaring in an exponential function shape toward the channel waveguidearray.
 8. An optical communication system comprising a plurality ofnodes connected by transfer lines into a loop form, wavelengthmultiplexed and demultiplexed light signals being transferred along theloop form transfer line, the nodes each including a first arraywaveguide grating for demultiplexing the multiplexed light signal intolight signals of different wavelengths and a second array waveguidegrating for multiplexing the demultiplexed light signals of thedifferent wavelengths, wherein the first array waveguide gratingincludes a predetermined substrate, a first and a second channelwaveguide for light wave transfer on the substrate, a channel waveguidearray having a plurality of component waveguides having lengthsprogressively increasing with a predetermined difference betweenadjacent ones of the waveguides on the substrate, a first slab waveguidefor connecting the ends of the first channel waveguides and one end ofthe channel waveguide array via a waveguide part having a first shape onthe substrate, a second slab waveguide for connecting one end of thesecond channel waveguides and the other end of the channel waveguidearray via a waveguide part having a second shape on the substrate, andat least the open part of each of the first channel waveguides on theside of the first slab waveguide or the open part of each of the secondchannel waveguides with respect to the second slab waveguide has a shapepart flaring in an exponential function shape represented by a functionof a degree higher than the second degree toward the channel waveguidearray.
 9. The optical communication system according to claim 8, whereinthe flaring shape part represented by the function of a degree higherthan the second degree has such a convex shape that when frequencymultiplexed Gaussian waveform light waves pass through their waveguides,their characteristics line in a rage between boundary ranges ofcharacteristics with respect to the transmission width and thecross-talk when they pass through the rectangular waveguides and seconddegree function shape waveguides.
 10. An optical communication systemcomprising a plurality of nodes connected by transfer lines into a loopform, wavelength multiplexed and demultiplexed light signals beingtransferred along the loop form transfer line, the nodes each includinga first array waveguide grating for demultiplexing the multiplexed lightsignal into light signals of different wavelengths and a second arraywaveguide grating for multiplexing the demultiplexed light signals ofthe different wavelengths, wherein the first array waveguide gratingincludes first and second channel waveguides for light wave transfer, achannel waveguide array having a plurality of component waveguideshaving lengths progressively increasing with a predetermined differencebetween adjacent ones of the waveguides, a first slab waveguide disposedbetween the first channel waveguides and one end of the channelwaveguide array, a second slab waveguide disposed between the secondchannel waveguides and the other end of the channel waveguide array, atleast the open part of each of the first channel waveguides on the sideof the first slab waveguide or the open part of each of the secondchannel waveguides on the side of the second slab waveguide has an openend with an opening width greater than the waveguide width of the firstor second channel waveguides, and the shape directed from the stem partof the open part toward the open end is found on the inner side ofrectangular shape of the opening width and on the outer side of a seconddegree curve connecting the stem part and the open end.